From StrategyWiki, the free strategy guide and walkthrough wiki

Go up the room, past the silver cube up to and through the shady arch in the back wall.

Enter the next arch directly in front to enter a room where a big spiked ball traverses the room in a clock-wise direction. There are three waves of enemies, use bombs for one hit kills on the gray mudmen, but beware they tetravate or quarternate (they split into four) and then explode upon the killer blow. Yes three waves, not two, as a gamer weak of heart may think (like me when I first had a go) so don't panic when the door doesn't open and no switch appears instantly after wave two.

If you wish to explore the area to the right of the room with the spiked ball, be forewarned that the two chests contain medicinal herbs, only if desperate or obstinatley obsessive should you bother, a green herb plant (that may relinquish a herb on being picked up aloft and then chucked) as any times as needed where Alundra exits and re enters is about four or five puzzles away.

The third wave are like Sara's true form (remember Mr Bodacious — Bonaire's nightmare) in that they teleport but follow the shadow and they are easy prey for the Releaser.

Flick the switch to remove the silver cube referred to in Line 1 and then double back to the newly opened stairs.

[edit] Main puzzles

The 16 switch room will leave you cross unless you see how to make all switches green across the room.

The three descending spiked walls, learn where to fall to get ice cube. No pun big or otherwise intended via hip hop, this ice cube is rapper free.

[edit] Tormenter (Boss that devoured Kline's soul)

To sort this one out, chuck a bomb between yourself and it (but keep at least two steps from the bomb if possible) do this when it closes it's mouth and then as soon as the 1st one lands lift another above Alundra's head and as bomb X ignites launch bomb Y.

NB Septimus announces at a timely juncture that "fanatic" Giles (his sister's words not mine) cannot afford any serious mental damage, 'lest the reaper claim the debt thereon' (my words not his sister's). So try not to let the tormentor get ruminative with Giles, after all what would Buffy do without him.